The present invention relates to a method for controlling a hydrocarbons production installation. The method is applied to an installation comprising one or more hydrocarbons production strings.
Document FR 2 783 557 relates to a method for controlling a liquid and gaseous hydrocarbons production well activated by gas injection. The well comprises at least one production string equipped with an outlet choke with an adjustable or variable aperture. Pressurized gas, the flow rate of which is adjustable using a control valve, is injected into the annular string. The method comprises a start-up phase that consists of carrying out the following series of steps:                a step of initiating the hydrocarbons production,        a step of ramping up to production speed,        
followed by a production phase. During these phases, the outlet choke of the production string concerned (if the installation comprises several strings, the various chokes are operated) and the control valve are operated in order to maintain the stability of the flow rate of the hydrocarbons produced.
Said document describes a sequential control operation of the well valves. The drawback of such a sequential operation is the difficulty of adapting to the instabilities of the well, resulting in an undesirable variation in the liquid flow rate or gaseous flow rate at the wellhead. Such instabilities prolong the start-up time of the well. Furthermore, a drawback of the operation described in said document is that certain phases of the operation are governed by too long a time lag, which delays the production phase.
Document EP 0 756 065 describes a system for controlling the hydrocarbons production using a production string that extends into a production well. Gas is injected at the bottom of the string. The system comprises a choke with a variable or adjustable aperture for adjusting the flow of hydrocarbons in the production string and comprises a control module for dynamically controlling the choke aperture. The system described is a dynamic choke aperture adjustment system, but the drawback of such a system is that the start-up of the well is not carried out under optimum conditions. In particular, the system described does not allow appropriate management of the occurrence of hydrocarbons plugs at the well start-up. In effect, a system operating solely continuously does not allow free action on the gas production and injection chokes to allow plugs to be expelled. Similarly, if such a continuous system drifts, it is not possible to make good by a stabilizing mode of operation.
Document U.S. Pat. No. 6,595,294 describes a method for controlling the production flow rate of a well. The well comprises a production string with at least one production choke and gas injection means comprising at least one gas injection choke. At least one of the chokes is controlled continuously using a model-based control system. The system comprises a stabilization controller based on a dynamic return of at least one of the elements chosen from a measurement of pressure, temperatures or flow rates in the well. These pressures, temperatures and flow rates are effectively stabilized by the model-based control system at specified operation points, even if the specified operation point is unstable in an open loop. The operation described is a dynamic adjustment operation but is based on the use of a mathematical model.
However, an operation based on the use of a model has drawbacks. One drawback is that it is difficult for a model to take account of the temperature of the valves at the moment of start-up. This temperature has an effect on the manner of start-up of the well. Also, a further drawback is that it is difficult for a model to take account of the state of the fluids around the well, which are unpredictable. These drawbacks thus do not allow start-up of the well under conditions as close as possible to the actual well conditions.
The document Automatic Control of Unstable Gas Lifted Wells (SPE 56832, by Bard, Lemetayer et al, 1999) describes methods centred on manipulation of the production and gas injection valves. In this article, the methods are described as alternatives and are the methods forming the subjects of patents U.S. Pat. No. 6,595,294 and FR 2 783 557.
Other methods are disclosed in documents WO 2006/067151, WO 00/75477, U.S. Pat. No. 5,413,175, US 2006/041392, “Cascade control of unstable systems with application to stabilization of slug flow” by Espen Storkaas and Sigurd Skogestad (IFAC-SYMPOSIUM ADCHEM), and “Stabilization of gas lifted wells based on state estimation” by Gisle Otto Eikrem et al. (IFAC), the contents of each of which are hereby incorporated by reference. While these prior art methods have achieved a varying degree of success, the art is constantly in need of a method for controlling a hydrocarbons production installation that solves all or some of the above problems.